1. Field of the Invention
The present invention relates to a wire electric discharge machine having a function of improving positioning accuracy, which is affected and deteriorated by the installation state of the machine, a machining liquid weight, a workpiece weight, and the like.
2. Description of the Related Art
Usually, in an assembly plant, a wire electric discharge machine uses an expensive high-precision measurement device such as a laser interferometer (see Japanese Patent Application Laid-Open No. 62-271633) and a pitch master (see Japanese Patent Application Laid-Open No. 3-161247) to measure the positioning accuracy of each drive axis, and compensates the pitch error.
In FIG. 21, (a) is a figure illustrating a reference state in which the pitch error is compensated in an assembly plant, (b) is a figure illustrating a state in which a bed is bent due to the effects of the machining liquid and the workpiece, and (c) is a figure illustrating a state in which the strength of the floor is weak, and the floor is dented, so that the bed is bent. In the state (b), the central portion of the bed is dented, and a distance between two points is shorter than that in the state (a). In the state (c), both sides of the bed are dented, and a distance between two points is longer than that in the state (a). It should be noted that the bed means a mechanism unit for carrying and holding a table moving mechanism in the wire electric discharge machine. FIG. 22 is a figure illustrating pitch errors in the state (a), (b) and (c) of FIG. 21.
Since expensive high-precision measurement devices such as the laser interferometer and the pitch master cannot be used in machining liquid, the positioning accuracy is measured while the machining liquid is not accumulated in a machining tank in an assembly plant.
However, during actual machining of a workpiece, it is necessary to accumulate the machining liquid to a level equal to or higher than the height of the upper surface of the workpiece, and the machine may be affected by the machining liquid weight and the workpiece weight, and therefore the pitch accuracy of the workpiece may not fit within a reference value. When the machine is shipped from the assembly plant and brought and installed in a user's plant, the amount of deflection of the machine is different due to difference of the strength of the floor and difference of the leveling state of the machine between the assembly plant and the user's plant, and therefore the positioning accuracy may change. Therefore, in an ideal case, after the machine is installed in the user's plant, it is necessary to compensate the pitch error again using a measurement device such as a laser interferometer and a pitch master. However, this requires an expensive measurement device, and the installation and the method for using the measurement device are difficult. Therefore, in reality, it is difficult for the user to measure the positioning accuracy.